The present invention relates to the automated manufacturing of fiber optic couplers.
Overclad fiber optic couplers are a type of fused fiber coupler wherein the coupling region is enclosed within a layer of matrix glass which strengthens and encloses the coupling region. To form an overclad fiber optic coupler, the stripped portions of a plurality of fibers are inserted into the bore of a glass capillary tube to form a coupler preform. The tube bore has enlarged funnel-shaped end portions that facilitate the insertion of optical fibers. The midregion of the coupler preform is heated to collapse the tube onto the fibers; the coupler preform is then stretched until the desired coupling characteristics are obtained. Various types of overclad fiber optic couplers and methods of making such couplers are disclosed in U.S. Pat. Nos. Re 35,138, 4,902,324, 4,979,972, 5,011,251, 5,251,276 and 5,268,014. The methods disclosed in these patents include many manual operations.
In accordance with conventional practice, the manually operated fiber draw apparatus has been oriented such that the tube is vertically positioned. The fibers have been inserted into the tube either on-line or off-line. The off-line fiber insertion process (U.S. Pat. No. 4,902,324) requires that the fibers be tacked to the tube to prevent the fibers from moving with respect to the tube during the step of transferring the coupler preform to the coupler draw apparatus. The tacking glue can cause problems in the resultant coupler. Moreover, the off-line method requires additional steps to transfer the tube to the draw apparatus. The previously employed methods of inserting fibers into the tube either on-line or off-line have been tedious, time consuming processes that are sensitive to the manipulations of each operator. This can affect process reproducibility and thus the optical characteristics of the couplers.
Optical fibers must be prepared prior to inserting them into the tube. The protective coating is removed from the portion of the fiber that is to be positioned within the tube during the coupler drawing operation. If the bare portion of the optical fiber is at the end of the fiber, it is preferred that it be provided with a low reflectance termination. An off-line process for forming such a termination is disclosed in U.S. Pat. Nos. 4,979,972 and 5,011,251. Also, the bare fiber portions must be free from contamination. Manual performance of these fiber preparation steps is time consuming and is subject to the particular manipulations of the operator.
During the stripping of coating from the fibers, the termination of fibers, and the insertion of the stripped portions of fibers in the overclad tube, the fibers must be precisely positioned.
In the manual technique for making overclad fiber optic couplers, the fibers were threaded through the glass tube, the tube was clamped into the draw apparatus. Thereafter, the fiber pigtails extending from the glass tube were inserted through vacuum attachments which were then affixed to the ends of the tubes. Such vacuum attachments are unsuitable for an automated apparatus for manufacturing fiber optic couplers. A preferred heat source for forming overclad fiber optic couplers has been a ring burner that directs flames inwardly toward the glass tube. Heretofore, the glass tube has been manually inserted through the ring burner, and its ends were then clamped. Such a burner is not suitable for use in a fully automated apparatus.
In an automated fiber optic coupler manufacturing process, couplers can be made at a greater rate than they could be made by the aforementioned manual process. The heat source must be activated during the stretching of each coupler. This tends to cause the temperature of certain parts of the apparatus near the heat source to become hotter than they did in the manual process. Some of those apparatus parts and the coupler epoxy can be damaged by the higher temperature or can be dimensionally altered whereby process reproducibility is affected. Precautions must be taken to avoid such heat induced damage.
After the coupler has been formed by stretching the overclad tube and fibers, a glue such as an ultraviolet (UV) curable epoxy is inserted into the uncollapsed ends of the tube bore to provide the fibers with pull strength. Conventional off-line epoxy applying and curing techniques are not suitable for use in a fully automated coupler making process since they do not result in the application of a sufficient amount of epoxy into both ends of the bore, and since they are time consuming processes.